spark join

https://jaceklaskowski.gitbooks.io/mastering-apache-spark/content/spark-sql-joins.html

https://acadgild.com/blog/what-is-join-in-apache-spark/

http://spark.apache.org/docs/latest/programming-guide.html

https://www.iteblog.com/archives/1566.html

http://www.agildata.com/apache-spark-2-0-api-improvements-rdd-dataframe-dataset-sql/